A hybrid vehicle has an internal combustion engine such as a gasoline engine and a diesel engine (as will be simply called the “engine” hereinafter), and a motor having a generating function. In the hybrid vehicle, a fuel efficiency of can be improved and a gas emission can be reduced by operating the engine and the motor in an optimal way. One example of the hybrid vehicle of this kind is disclosed in Japanese Patent Laid-Open No. 2008-265598.
According to the teachings of Japanese Patent Laid-Open No. 2008-265598, the hybrid vehicle comprises an engine, a first motor, a second motor, and a power distribution device such as a planetary gear unit having three rotary elements. A rotation of an output shaft of the engine is halted by a clutch. The first motor is connected to the output shaft of the engine through the power distribution device, and the second motor is connected to driving wheels. The engine, the first motor, the second motor, and the clutch are controlled in accordance with a required driving force. In the hybrid vehicle, a motor mode can be established by activating the first and the second motors while using the power distribution device as a speed reducing device or a speed increasing device by bringing the clutch into engagement to halt the output shaft of the engine.
In the hybrid vehicle taught by Japanese Patent Laid-Open No. 2008-265598, specifically, an operating mode can be selected from “an engine mode” in which the vehicle is powered by the engine, “a single-motor mode” in which the vehicle is powered only by the second motor, and “a dual-motor mode” in which the vehicle is powered by both the first and the second motors while reducing the engine speed to zero by bringing the clutch into engagement. In the hybrid vehicle, therefore, fuel efficiency can be improved by appropriately selecting the operating mode to operate the engine and the motors in an optimal way.
However, on the occasion of shifting the operating mode from “the engine mode” in which the vehicle is powered by the engine to “the dual-motor mode” in which the vehicle is powered by both first and second motors, the engine is stopped while temporarily propelling the vehicle only by the second motor, and then the clutch is brought into engagement to establish “the dual-motor mode”. Indeed, a drive torque generated only by the second motor is smaller than that generated under “the engine mode”, and hence the drive torque to propel the vehicle drops temporarily when shifting from “the engine mode” to “the dual-motor mode”. In this situation, if the drive torque drops significantly against the driver's will, the driver may feel a shock and discomfort.